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Bright Shiny Beetles

The beautiful iridescent colors found on the wings of butterflies and on the bodies of beetles have attracted the attention of brilliant minds over the past centuries, starting with Newton, who understood that these colors must involve “thin film structures.” In 1911 Michelson described the metallic appearance of these beetles, and in the late 1960s Neville and Caveney discussed the optical properties in the context of cholesteric liquid crystals. Sharma et al. (p. 449; see the Perspective by Vukusic) examined the metallic green beetle Chrysina gloriosa, which selectively reflects left circularly polarized light when illuminated with unpolarized light. The underlying cellular structure of the beetle exoskeleton is organized primarily in a hexagonal pattern, with variations in the pentagonal and heptagonal arrangements depending on the local curvature. Thus, the ordering of the cells in concentric, nested arcs is indeed analogous to the ordering of the molecules in a cholesteric (or chiral nematic) liquid crystal.

Abstract

The iridescent metallic green beetle, Chrysina gloriosa, which selectively reflects left circularly polarized light, possesses an exoskeleton decorated by hexagonal cells (~10 μm) that coexist with pentagons and heptagons. The fraction of hexagons decreases with an increase in curvature. In bright field microscopy, each cell contains a bright yellow core, placed in a greenish cell with yellowish border, but the core disappears in dark field. With use of confocal microscopy, we observe that these cells consist of nearly concentric nested arcs that lie on the surface of a shallow cone. We infer that the patterns are structurally and optically analogous to the focal conic domains formed spontaneously on the free surface of a cholesteric liquid crystal. These textures provide the basis for the morphogenesis as well as key insights for emulating the intricate optical response of the exoskeleton of scarab beetles.